Help for a friend wanted

Although this isn't Audi specific, I was wondering if someone could
give me some suggestions on what my friend can do with being overcharged
for service done on his car.
Here is the basics... This past week I went to Sugarbush VT for vacation,
and two of my friend's went with us. They had to leave on Thurs, and we
left today, Saturday. Anyway, when they got to Connecticut, the car broke
down. (I'm not sure the make of the car, but it is American) They went to
a local repair shop and was told the problem was the fuel pump. It was
replaced Friday, with a price of just under $700. Yes, that's $700. $400
for the part, and $300 for labor. Then an hour after they left, it broke
down again. (Oh, they had to pay cash) So they called the guy up and he
sent a tow truck to take them the rest of the way home. (No charge (south
Jersey)
The guy told them (I assume sometime last night) that they could send
the pump back and be refunded the $400, which they are going to do.
My question is how should they go about this & who should they contact
to get ALL their money back? What should they do before sending the part
back? I worry that if they just send the part back, the guy might not
send the money. What a mess... If anyone could help, I would GREATLY
appreciate it.
STEve
-steved@panix.com